1. Field of the Invention
The present invention relates to papermakers belts, in general, and to dryer felts incorporating antistatic yarns, in particular.
2. Background of the Prior Art
The problem of static electricity is well-known in the papermaking industry. A spark discharge from highly charged materials is a common occurrence and is thought to have been the cause of numerous unexplained fires. The incidence of static problems in the papermaking industry has tended to increase primarily due to the introduction of new polymers and fiber materials which are decidedly hydrophobic.
Dryer felts, employed in the dryer section of a papermaking machine, are particularly prone to the accumulation of electrostatic charge. In the dryer section, the dryer felt or fabric passes over a number of machine rolls which cause a separation of charge while the rolls and the fabric are in contact. When the dryer fabric leaves a roll, both the roll and the fabric surfaces hold a free electrical charge. The roll, because it is made of metal, has good conductivity thereby losing or markedly reducing the free charge. The fabric, on the other hand, is a poor conductor and retains the free charge. At the next roll, the process is repeated, and, again, the fabric surface holds a free charge in addition to the free charge it previously held. Build up of static under these conditions is therefore fairly rapid.
It is believed that, in general, separation of charge between two insulating materials occurs at their points of contact and is usually increased with rubbing. While the two surfaces are in contact, there is a balance between adjacent layers of charge of opposite polarity. However, when the surfaces are separated, a residual free charge remains on each surface, which causes static problems. In a dryer fabric, the accumulation of electrostatic charge is attributed to frictional rubbing forces encountered by the fabric during contact with the various machine rolls in the dryer section of the papermaking machine.
Such electrostatic accumulations in the dryer fabric can increase the tendency of the fabric to attract and retain foreign matter whereby the fabric tends to clog with dirt. Electrostatic accumulations can also lead to a spark discharge and an attendant fire risk. Sheet separation or sheet follow can also be caused by electrostatic accumulations. In sheet separation, the paper sheet leaves the dryer fabric when it should be travelling with it, whereas, in sheet follow, the paper sheet will not leave the dryer fabric to transfer to another section of the papermaking machine.
The problem of electrostatic accumulation is enhanced in dryer fabrics woven from or including hydrophobic yarns, such as polyesters, nylon, or acrylics, as compared to dryer fabrics made from hydrophilic materials, such as cotton and other natural fibers.
Various attempts have been made to resolve static problems in dryer fabrics. One early attempt is discussed in U.S. Pat. No. 2,949,134 to Hindle et al. In this patent, various yarns with different static charge polarity characteristics are woven into a fabric so that the differing polarities may interact with each other to negate static charge.
Another approach is to increase moisture content in the area where the dryer fabric contacts the machine rolls. However, this approach has a primary shortcoming in that the object of the dryer section is to remove the remaining excess moisture contained in the paper web being dried.
An additional approach, commonly used today, is to apply an anti-static finish to the surface of the yarn. This is generally done during the manufacture of the yarn in the case of synthetic yarns. However, such finishes are generally water soluble and are removed in subsequent paper drying processing, thus, severely limiting the life of the anti-static properties of the dryer fabric.
There is thus a need for a dryer fabric in which the problem of electrostatic charge accumulation is eliminated without any sacrifice in fabric performance characteristics and without a substantial increase in cost. The present invention is directed toward filling that need.